Laser-diode-pumped solid-state laser using index-guided type multi-transverse mode broad area laser

Abstract: A laser-diode-pumped solid-state laser in which a solid-state laser crystal (13) is pumped by a laser beam emitted from a laser diode (11). An index-guided type multi-transverse mode broad area laser (11) having a single optical waveguide is used as the laser diode. This leads to reduced output variations and noise of the laser. Preferably, the front reflectivity of the laser (11) with respect to the oscillation wavelength of the same should be more than 20%.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a laser-diode-pumped solid-state laser, and more particularly to a laser-diode-pumped solid-state laser in which output variations and noise are reduced. Description of the Prior Art Gain-guided broad area laser diodes for pumping Nd:YAG solid-state lasers are suggested from "High-Power Operation of Broad-Area Laser Diodes with GaAs and AlGaAs Single Quantum Wells for Nd:YAG Laser Pumping" by k. Shigihara et. al. in IEEE Journal of Quantum Electronics, vol. 27, no.6, June 1991, p. 1537-1543 and from "High-Power Operation of AlGaAs SQW-SCH Broad Area Laser Diodes for Nd:YAG Solid-State Laser Pumping" by Y. Nagai et. al. in IEEE Photonics Technology Letters, vol.1, no.2, Feb. 1991, p.97-99. An index-guided laser diode is known from "Spatial Mode Structure of Index-guided Broad-Area Quantum-Well Lasers" by C.J. Chang-Hasnain et.al. in IEEE Journal of Quantum Electronics, vol. 26, no.10, Oct.1990, p.1713-1716. As disclosed in, for example, U.S. Patent No. 4,656,635, a laser-diode-pumped solid-state laser is well known, wherein a solid-state laser crystal doped with rare earth elements such as Neodymium is pumped with a laser diode (a semiconductor laser). In this laser-diode-pumped solid-state laser, an optical output thereof is detected by a photodetector, and a drive current of the laser diode is subjected to a feedback control on the basis of an output obtained from that detection. Thereby, so-called APC (automatic power control) is effected for maintaining an optical output at a predetermined value. Thus, the laser-diode-pumped solid-state laser is often used while being subjected to APC. In this type of laser-diode-pumped solid-state laser, a multi-transverse mode broad area laser is often used as a laser diode in order to obtain a high pump power. More particularly, the multi-transverse mode broad area laser, conventionally used as such a pumping source, is of the gain-guided type which forms a guide mode using a current gain. In the conventional laser-diode-pumped solid-state laser that uses the gain guide type multi-transverse mode broad area laser as a pump source, sharp output variations (so-called kinks), resulting from variations in drive current of the laser diode itself, and minute output variations and noise, caused by returning light which is reflected from a solid-state laser crystal or the like, occur. This may lead to a considerably deteriorated linearity of drive current vs. optical output characteristics of the laser diode. In such a case, it is impossible to suppress output variations of the solid-state laser or noise even when the laser is subjected to the aforementioned APC. What is worse, this causes the APC circuit to oscillate, which in turn causes new noise. This problem is particularly noticeable when the environmental temperature around the laser-diode-pumped solid-state laser varies. SUMMARY OF THE INVENTION In view of the foregoing observation, the object of the present invention is to provide a laser-diode-pumped solid-state laser in which output variations and noise can be reduced. To this end, according to this invention, there is provided a laser-diode-pumped solid-state laser as defined in claim 1. In an embodiment of this laser-diode-pumped solid-state laser, the index-guided type multi-transverse mode broad area laser has a front reflectivity of more than 20% with respect to a wavelength thereof. In the index guide type multi-transverse mode broad area laser is a laser diode provided with a wide three-dimensional optical waveguide which confines a number of transverse modes. When this laser diode is used as a pump source, it becomes difficult for sharp output variations called kinks to occur. Also, when the laser diode having a high front reflectivity (a reflectivity of more than 20% is more effective) is used as this multi-transverse mode broad area laser, minute output variations and noise resulting from the foregoing returning light are further reduced. As a result of this, in the laser-diode-pumped solid-state laser of the present invention, output variations and noise of the solid-state laser beam are prevented. In addition, if output variations and noise of the laser diode used as a pump source are suppressed in the manner as mentioned above, superior control effects will be obtained when the foregoing automatic power control is carried out. Moreover, it is possible to solve the problem of the oscillation of the APC circuit causing new noise. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side view of a laser-diode-pumped solid-state laser according to a first embodiment of the present invention; Figure 2 is a graph showing the relationship between a laser diode drive current and an output of a second harmonic wave in the laser shown in Figure 1; Figure 3 is a schematic side view of a conventional laser-diode-pumped solid-state laser that is an example which will be compared to the first embodiment shown in Figure 1; Figure 4 is a graph showing the relationship between a laser diode drive current and an output of a second harmonic wave in the laser shown in Figure 3; Figure 5 is a schematic side view showing a laser-diode-pumped solid-state laser having a resonator removed; Figure 6 is a graph showing the relationship between a laser diode drive current and an optical output when the front reflectivity of a laser diode of the laser shown in Figure 5 is 7.5%; Figure 7 is a graph showing the relationship between a laser diode drive current and an optical output when the front reflectivity of a laser diode of the laser shown in Figure 5 is 20%; Figure 8 is a graph showing the relationship between a laser diode drive current and an optical output when the front reflectivity of a laser diode of the laser shown in Figure 5 is 30%; Figure 9 is a graph showing the relationship between a laser diode drive current and an optical output when a laser diode is driven in a free-running state; Figure 10 is a schematic side view of a conventional laser-diode-pumped solid-state laser that is an example which will be compared to the laser shown in Figure 5; Figure 11 is a graph showing the relationship between a laser diode drive current and an optical output in the laser shown in Figure 10; Figure 12 is a graph showing the relationship between a laser diode drive current and an optical output when a laser diode used in the laser shown in Figure 10 is rendered in a free-running state; and Figure 13 is a plot showing the relationship of output variations and noise to the front reflectivity of the laser diode of the laser shown in Figure 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the accompanying drawings, the present invention will be described in detail hereunder. Fig. 1 shows a laser-diode-pumped solid-state laser according to the invention, and it is composed of: a laser diode 11 which emits a laser beam 10 as a pump beam; a condenser lens 12 consisting of, for example, a distributed index lens that converges the laser beam 10 of diverging rays; a YAG crystal 13 (hereinafter referred to as Nd:YAG crystal) which is a solid-state laser medium doped with Neodymium (Nd); a resonator mirror 14 disposed (on the right side in the drawing) in front of this Nd:YAG crystal 13; a KN crystal 15 interposed between the resonator mirror 14 and the Nd:YAG crystal 13; and an etalon 16 interposed between the KN crystal 15 and the resonator mirror 14. Moreover, in front of the resonator mirror 14, a blue color transmitting filter 17 which permits the transmission of a second harmonic wave 21 which is emitted in the manner as will be mentioned below, and a partially transmitting mirror 18 which causes a part of the second harmonic wave 21 having passed through the filter 17 to be reflected and bifurcated. The second harmonic wave 21 thus bifurcated is detected by a photodetector 19 such as a photodiode are provided. An index-guided type multi-transverse mode broad area laser (having an oscillation wavelength of 808.5 nm) having a single optical waveguide is used as the laser diode 11 which acts as a pump source. The front reflectivity of this laser diode 11 with respect to a wavelength of 808.5 nm is 30%. On the other hand, the Nd:YAG crystal 13 has an Nd concentration of, for example, 1 at.% and a length of 1 mm. Neodymium atoms are pumped by an incoming laser beam 10 having a wavelength of 808.5 nm, as a result of which the Nd:YAG crystal 13 emits a laser beam 20 having a wavelength of 946.2 nm. This laser beam 20 is rendered in a single longitudinal mode by the etalon 16, and it is converted into a blue second harmonic wave 21 having a half wavelength, i.e., 473.1 nm by the KN crystal 15 which is a non-linear optical material. This second harmonic wave 21 transmits the appropriately coated resonator mirror 14, and leaves to outside of the resonator. A part of the second harmonic wave 21 is introduced to the photodetector 19 in the manner as mentioned above, and an optical output L B thereof is detected by the photodetector 19. An APC circuit 22 controls a drive current I of the laser diode 31 on the basis of an optical detection signal S outputted from the photodetector 19, whereby the optical output of the second harmonic wave 21 is maintained at a constant value. Figure 2 shows the relationship between the drive current I of the laser diode 11 and the optical output L B of the second harmonic wave 21 when automatic power control is not carried out in this embodiment. As illustrated in Figure 2, when the laser diode drive current I varies, neither distinguishing output variations nor noise occur in the second harmonic wave 21. It is deemed that this is because an appropriate linearity of the drive current vs. output characteristics of the laser diode 11 is obtained. In this case, a stable control effect will be obtained when automatic power control is effected. Comparative Example 1 To ascertain the effects of the present invention, the laser diode 11 used in the configuration shown in Figure 1 is replaced with a gain-guided type multi-transverse mode broad area laser 31 used in a conventional laser, whereby a comparative example shown in Figure 3 is manufactured. An oscillation wavelength of this gain-guided type multi-transverse mode broad area laser 31 is 808.5 nm, and the front reflectivity of the laser with respect to this oscillation wavelength is 15%. Figure 4 illustrates the relationship between the laser diode drive current I and the optical output L B of the second harmonic wave 21 when automatic power control is not carried out in the laser-diode-pumped solid-state laser of this comparative example 1. As shown in the drawing, distinguishing output variations arise in the second harmonic wave 21 as the laser diode drive current I varies. It is considered that this is because the linearity of the drive current vs. output characteristics of the laser diode is impaired. In this state, no stable automatic power control is obtained, and hence leaps in output are repeated. At C1 in a curve shown in Figure 4, the optical outputs L B are completely or substantially the same with respect to one laser diode drive current I. This is because the laser diode 31 itself oscillates, which will induce noise when automatic power control is effected. At C2 in the curve shown in Figure 4, the optical outputs L B having the same value are obtained with respect to two laser diode drive currents I. In this state, the APC circuit 22 oscillates when automatic power control is carried out, which in turn causes circuit noise. Referring to Figure 5, an example will be described in which, in order to examine the characteristics of the laser diode, the resonator is removed from the entire construction. The laser-diode-pumped solid-state laser of this embodiment is made up of: the laser diode 11 which emits the laser beam 10 as a pump beam; the condenser lens 12 which converges the laser beam 10; and the Nd:YAG crystal 13 which is pumped by the laser beam 10. As with the previous embodiment, the index-guided type multi-transverse mode broad area laser having a single optical waveguide (having an oscillation wavelength of 808.5 nm) is used as the laser diode 11. Three types of laser are used as this laser diode 11, each having the front reflectivity of 7.5%, 20% and 30% with respect to the wavelength of 808.5 nm. In each case, an optical output L LD of the laser beam 10 having passed the Nd:YAG crystal 13 is measured by a power meter 40. The reflectivity of a light entrance end surface 13a of the Nd:YAG crystal 13 with respect to the wavelength of 808.5 nm was adjusted to 5%. The relationship between the drive current I and the optical output L LD of the laser diode 11 in each case is shown respectively in Figures 6, 7 and 8. While the condenser lens 12 and the Nd:YAG crystal 13 are removed from the construction shown in Figure 5, the laser diode 11 is driven in a free-running state. The optical output L LD of the laser 10, which is emitted in this state, is directly measured by the optical power meter 40. Figure 9 illustrates the relationship between the drive current I and the optical output L LD of the laser diode 11 obtained at this time. In this case, the index-guided type multi-transverse mode broad area laser having the front reflectivity of 30% with respect to the wavelength of 808.5 nm is used as the laser diode 11. Comparative Example 2 A comparative example shown in Figure 10 is manufactured by replacing the laser diode 11 used in the construction shown in Figure 5 with the gain-guided type multi-transverse mode broad area laser 31 used in a conventional laser. The oscillation wavelength of this gain-guided type multi-transverse mode broad area laser is 808.5 nm, and the front reflectivity of the same with respect to this oscillation wavelength is 15%. Even in this case, the reflectivity of the light entrance end surface 13a of the Nd:YAG crystal 13 with respect to the oscillation wavelength 808.5 nm is also 5%. In this comparative example 2, the optical output L LD of the laser beam 10 having passed the Nd:YAG crystal 13 is measured by the optical power meter 40. Figure 11 shows the relationship between the drive current I and the optical output L LD of the laser diode 31 obtained at this time. While the condenser lens 12 and the Nd:YAG crystal 13 are removed from the construction, the laser diode 31 is driven in a free-running state. The optical output L LD of a laser beam 30 which is emitted from the laser diode 31 is directly measured by the optical power meter 40. Figure 12 shows the relationship between the drive current I and the optical output L LD of the laser diode 31 obtained at this time. In Figures 6, 7, 8, 9, 11, and 12, the optical output on the longitudinal axis is represented by a relative value. However, in Figures 6, 7, 8 and 11 which show the optical output L LD of the laser beam having passed the Nd:YAG crystal 13, absolute values in the full-scale longitudinal axis are equally shown. On the other hand, in Figures 9 and 12 which show the optical output L LD obtained when the diode is driven in the free-running state, absolute values in the full-scale longitudinal axis are equally represented. As shown in Figure 12, when the gain-guided type multi-transverse mode broad area laser 31 is driven in a free-running state, large output variations, i.e., so-called kinks arise as the drive current I varies. Contrary to this, when the index-guided type multi-transverse mode broad area laser 11 is driven in a free-running state, no kinks appear as shown in Figure 9. As previously mentioned, the characteristics shown in Figure 9 are obtained when the front reflectivity of the laser diode 11 is 30%. Similarly, in the case of the laser diode having either the front reflectivity of 7.5% or 20%, no kinks appear. In the construction (the second comparative example), shown in Figure 10, using the gain-guided type multi-transverse mode broad area laser 31, large output variations due to kinks appear as shown in Figure 11, and minute output variations are also observed. These minute output variations are caused by the influence of returning light. On the other hand, in the construction shown in Figure 5 in which the index-guided type multi-transverse mode broad area laser 11 is used, as a matter of course, no large output variations due to the foregoing kinks appear as shown in Figures 6, 7 and 8. Moreover, as explicitly shown in Figures 6, 7 and 8, there is tendency of minute output variations, caused by the influence of returning light, to be smaller as the front reflectivity of the laser diode 11 is increased. Figure 13 shows results of the measurement of output variations and noise of the laser beam 10 in each of the cases where the front reflectivity of the laser diode 11 is 7.5%, 20% and 30%. In these cases, the temperature of the atmosphere is varied in the range of 10 - 40 °C at a variation rate of 60°C/time. As shown in Figure 13, both output variations and noise evidently become smaller as the front reflectivity is increased. Particularly, when the front reflectivity is more than 20%, the output variations and noise sharply drop. Thus, the explanations have been given of the embodiments using the Nd:YAG crystal as the solid-state laser crystal. However, the present invention is applicable to laser-diode-pumped solid-state lasers other than the foregoing types. In such a case, the same effects as mentioned in the above will be obtained. Several embodiments of the invention have now been described in detail. It is to be noted, however, that these descriptions of specific embodiments are merely illustrative of the principles underlying the inventive concept. It is contemplated that various modifications of the disclosed embodiments, as well as other embodiments of the invention will, without departing from the scope of the invention as defined by the claims, be apparent to persons versed in the art.

Claims: A laser-diode-pumped solid-state laser in which a Nd:YAG laser crystal (13) is pumped by a laser beam (10) emitted from a multi-transverse broad area laser diode (11) having a single optical waveguide as said laser diode, wherein said laser diode (11) is of an index-guided type, and further comprising an automatic power control circuit for effecting automatic power control of the laser diode. A laser-diode-pumped solid-state laser as defined in claim 1, wherein the index-guided type multi-transverse mode broad area laser (11) has a front reflectivity of more than 20% with respect to a wavelength thereof.

Inventors: HAYAKAWA TOSHIRO, HAYAKAWA, TOSHIRO, HAYAKAWA, TOSHIRO, C/O FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1994
Main classification: H01S3
IPC codes: H01S3, H01S5